In recent years, it has been strongly required to make copying machines small-sized and accordingly to provide compacter image projection devices including a projection lens. FIGS. 1 and 2 show an image projection device already proposed to meet this requirement for use in slit exposure type optical systems. The proposed device has a striplike projection lens having upper and lower cut portions in corresponding relation to the width of the slit and are thereby made small-sized, and a frame H for holding the projection lens L (see, for example, Unexamined Japanese Patent Publication SHO 61-87125).
However, it is extremely difficult to hold such a striplike projection lens in position without eccentricity. Stated more specifically, FIG. 3 shows a component lens L1 having a diameter d1 and held by a lens barrel H having curved surfaces R1 and R2. In this case, the lens barrel H has an inside diameter d2 which is greater than the diameter d1 of the lens L1. The lens L1 indicated in solid line is likely to be displaced to the broken-line position shown. Assuming that the upper and lower cut portions of the lens L1 have an opening angle of 2.theta., the translational eccentricity .DELTA.h then involved is expressed by the following equation. ##EQU1##
Now suppose the lens is 40 mm in diameter d1, the fitting tolerance thereof is 0 to-0.03 mm, the lens barrel is 40 mm in diameter d2 and the fitting tolerance thereof is +0.025 to 0 mm. The translational eccentricity .DELTA.h is 0.039 when 8 is 45 degrees or 0.055 when .theta. is 60 degrees. If the lens is in the usual circular form, the translational eccentricity .DELTA.h is expressed by (d2 -d1)/2 and is therefore 0.028. Thus, the lens L1 having the upper and lower cut portions is about 1.4 times as great as the usual circular lens in translational eccentricity at .theta.=45 degrees or is about twice as great at .theta.=60 degrees.
The eccentricity of the projection lens adversely affects the image forming performance thereof and consequently poses a serious problem. Especially, if the projection lens for copying machines has a small number of component lenses and a wide field angle and is therefore narrow in the allowable range of eccentricities, such that a slight eccentricity impairs the image forming performance.